Twenty-Eight Thousand Plants
The motion: “A dominant allele is the one most common in a population.”
Your opponent: Gregor Mendel · Course: Pearson Edexcel International GCSE Biology (4BI1) · Assessment objectives: AO1, AO2
It is the single most persistent error in genetics, and it survives because the word 'dominant' sounds like it should mean that.
What you are asked to argue
Defeat the motion, then defend a correct account of dominance, genotype and phenotype under questioning.
What the specification expects you to bring
- Genotype and phenotype, and the difference
- Homozygous and heterozygous
- Dominant and recessive alleles
- Punnett squares and the 3:1 and 1:1 ratios
- Why a recessive condition can be rare or common independently of its recessiveness
Positions that are defensible
Disagreeing with the figure is not an error. Any of these can score full marks if argued well:
- That co-dominance and incomplete dominance complicate the simple model, provided the learner is precise
- That in a specific population a dominant allele may happen to be common — the error is treating that as what 'dominant' means
Common misconceptions this debate is built to expose
- Wrong: Dominant means the allele most people have.
The correction: Dominant means that when one copy is present with a recessive copy, the dominant characteristic is the one expressed. Frequency is a separate question entirely. - Wrong: Two organisms that look the same have the same genotype.
The correction: A heterozygote and a homozygous dominant show the same phenotype and carry different genotypes. That is why the ratio in the second generation is 3:1 and not 1:0.
What passing it proves
The genotype/phenotype distinction used precisely, a counterexample that defeats the motion, and a correctly reasoned ratio.
Why them?
Read the life before you argue with it — the case you are about to meet was built by a real person, over a real career:
- The life of Gregor Mendel — Father of Genetics · Friar & Geneticist · 1822–1884